Control valves are used in process control systems to control conditions such as flow, pressure, temperature, and/or liquid level by fully or partially opening or closing in response to a signal received from one or more valve controllers. Typically, a valve controller is operatively coupled to or includes one or more sensors or switches disposed within the system, thereby allowing the valve controller to compare one or more “setpoints” to a corresponding “process variable” whose value is provided by the switches or sensors. The opening or closing of control valves is typically done automatically by electrical, hydraulic, or pneumatic actuators. In addition, positioners may be used to control the opening or closing of the actuator based on, for example, electric or pneumatic signals received from the valve controller.
In typical control valve assemblies, the one or more switches (such as proximity switches) or other sensors of the valve controller are adapted to detect targets (such as magnets) that are coupled to a portion of the valve (e.g., a valve stem) to determine one or more operational parameters of the control valve, such as the position of the closure member of the control valve. The one or more switches or other sensors are generally affixed to a stationary portion of the control valve, such as a portion of the bonnet, and the target is attached to a displaceable stem of the closure member. The one or more switches or other sensors are chosen for a unique application. For example, Namur sensors may be used when it is desired that a resistance output increases from low to high when a target is within a predetermined range of the Namur sensor. As another example, in environments in which flammable gas is present, the switches or other sensors must not output a voltage (or current) above a threshold value that is capable of igniting the flammable gas. Because unique switches and/or sensors (or circuits that include the switches and/or sensors) and are required for a correspondingly unique output type, a large inventory of switches is necessary to accommodate custom applications. In addition, if a different output type of output is desired, an entirely different switch or sensor must be purchased and installed.